1. Field of the Invention
The invention relates generally to the field of fixed cutter bits used to drill wellbores through earth formations.
2. Background Art
Rotary drill bits with no moving elements on them are typically referred to as “drag” bits. Drag bits are often used to drill a variety of rock formations. Drag bits include those having cutters (sometimes referred to as cutter elements, cutting elements or inserts) attached to the bit body. For example, the cutters may be formed having a substrate or support stud made of carbide, for example tungsten carbide, and an ultra hard cutting surface layer or “table” made of a polycrystalline diamond material or a polycrystalline boron nitride material deposited onto or otherwise bonded to the substrate at an interface surface.
An example of a prior art drag bit having a plurality of cutters with ultra hard working surfaces is shown in FIG. 1. The drill bit 10 includes a bit body 12 and a plurality of blades 14 that are formed on the bit body 12. The blades 14 are separated by channels or gaps 16 that enable drilling fluid to flow between and both clean and cool the blades 14 and cutters 18. Cutters 18 are held in the blades 14 at predetermined angular orientations and radial locations to present working surfaces 20 with a desired back rake angle against a formation to be drilled. Typically, the working surfaces 20 are generally perpendicular to the axis 19 and side surface 21 of a cylindrical cutter 18. Thus, the working surface 20 and the side surface 21 meet or intersect to form a circumferential cutting edge 22.
Nozzles 23 are typically formed in the drill bit body 12 and positioned in the gaps 16 so that fluid can be pumped to discharge drilling fluid in selected directions and at selected rates of flow between the cutting blades 14 for lubricating and cooling the drill bit 10, the blades 14 and the cutters 18. The drilling fluid also cleans and removes the cuttings as the drill bit rotates and penetrates the geological formation. The gaps 16, which may be referred to as “fluid courses,” are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit 10 toward the surface of a wellbore (not shown).
The drill bit 10 includes a shank 24 and a crown 26. Shank 24 is typically formed of steel or a matrix material and includes a threaded pin 28 for attachment to a drill string. Crown 26 has a cutting face 30 and outer side surface 32. The particular materials used to form drill bit bodies are selected to provide adequate toughness, while providing good resistance to abrasive and erosive wear. For example, in the case where an ultra hard cutter is to be used, the bit body 12 may be made from powdered tungsten carbide (WC) infiltrated with a binder alloy within a suitable mold form. In one manufacturing process the crown 26 includes a plurality of holes or pockets 34 that are sized and shaped to receive a corresponding plurality of cutters 18.
The combined plurality of surfaces 20 of the cutters 18 effectively forms the cutting face of the drill bit 10. Once the crown 26 is formed, the cutters 18 are positioned in the pockets 34 and affixed by any suitable method, such as brazing, adhesive, mechanical means such as interference fit, or the like. The design depicted provides the pockets 34 inclined with respect to the surface of the crown 26. The pockets 34 are inclined such that cutters 18 are oriented with the working face 20 at a desired rake angle in the direction of rotation of the bit 10, so as to enhance cutting. It will be understood that in an alternative construction (not shown), the cutters can each be substantially perpendicular to the surface of the crown, while an ultra hard surface is affixed to a substrate at an angle on a cutter body or a stud so that a desired rake angle is achieved at the working surface.
A typical cutter 18 is shown in FIG. 2. The typical cutter 18 has a cylindrical cemented carbide substrate body 38 having an end face or upper surface 54 referred to herein as the “interface surface” 54. An ultrahard material layer (cutting layer) 44, such as polycrystalline diamond or polycrystalline cubic boron nitride layer, forms the working surface 20 and the cutting edge 22. A bottom surface 52 of the ultrahard material layer 44 is bonded on to the upper surface 54 of the substrate 38. The bottom surface 52 and the upper surface 54 are herein collectively referred to as the interface 46. The top exposed surface or working surface 20 of the cutting layer 44 is opposite the bottom surface 52. The cutting layer 44 typically has a flat or planar working surface 20, but may also have a curved exposed surface, that meets the side surface 21 at a cutting edge 22.
Cutters may be made, for example, according to the teachings of U.S. Pat. No. 3,745,623, whereby a relatively small volume of ultra hard particles such as diamond or cubic boron nitride is sintered as a thin layer onto a cemented tungsten carbide substrate. Flat top surface cutters as shown in FIG. 2 are generally the most common and convenient to manufacture with an ultra hard layer according to known techniques. It has been found that cutter chipping, spalling and delamination are common failure modes for ultra hard flat top surface cutters.
Generally speaking, the process for making a cutter 18 employs a body of tungsten carbide as the substrate 38. The carbide body is placed adjacent to a layer of ultra hard material particles such as diamond or cubic boron nitride particles and the combination is subjected to high temperature at a pressure where the ultra hard material particles are thermodynamically stable. This results in recrystallization and formation of a polycrystalline ultra hard material layer, such as a polycrystalline diamond or polycrystalline cubic boron nitride layer, directly onto the upper surface 54 of the cemented tungsten carbide substrate 38.
It has been found by applicants that many cutters develop cracking, spalling, chipping and partial fracturing of the ultra hard material cutting layer at a region of cutting layer subjected to the highest loading during drilling. This region is referred to herein as the “critical region” 56. The critical region 56 encompasses the portion of the ultrahard material layer 44 that makes contact with the earth formations during drilling. The critical region 56 is subjected to high magnitude stresses from dynamic normal loading, and shear loadings imposed on the ultrahard material layer 44 during drilling. Because the cutters are typically inserted into a drag bit at a rake angle, the critical region includes a portion of the ultrahard material layer near and including a portion of the layer's circumferential edge 22 that makes contact with the earth formations during drilling.
The high magnitude stresses at the critical region 56 alone or in combination with other factors, such as residual thermal stresses, can result in the initiation and growth of cracks 58 across the ultra hard layer 44 of the cutter 18. Cracks of sufficient length may cause the separation of a sufficiently large piece of ultra hard material, rendering the cutter 18 ineffective or resulting in the failure of the cutter 18. When this happens, drilling operations may have to be ceased to allow for recovery of the drag bit and replacement of the ineffective or failed cutter. The high stresses, particularly shear stresses, can also result in delamination of the ultra hard layer 44 at the interface 46.
One type of ultra hard working surface 20 for fixed cutter drill bits is formed as described above with polycrystalline diamond on the substrate of tungsten carbide, typically known as a polycrystalline diamond compact (PDC), PDC cutters, PDC cutting elements, or PDC inserts. Drill bits made using such PDC cutters 18 are known generally as PDC bits. While the cutter or cutter insert 18 is typically formed using a cylindrical tungsten carbide “blank” or substrate 38 which is sufficiently long to act as a mounting stud 40, the substrate 38 may also be an intermediate layer bonded at another interface to another metallic mounting stud 40.
The ultra hard working surface 20 is formed of the polycrystalline diamond material, in the form of a cutting layer 44 (sometimes referred to as a “table”) bonded to the substrate 38 at an interface 46. The top of the ultra hard layer 44 provides a working surface 20 and the bottom of the ultra hard layer cutting layer 44 is affixed to the tungsten carbide substrate 38 at the interface 46. The substrate 38 or stud 40 is brazed or otherwise bonded in a selected position on the crown of the drill bit body 12 (FIG. 1). As discussed above with reference to FIG. 1, the PDC cutters 18 are typically held and brazed into pockets 34 formed in the drill bit body at predetermined positions for the purpose of receiving the cutters 18 and presenting them to the geological formation at a rake angle.
FIG. 3 shows a prior art PDC cutter held at an angle in a drill bit 10 for cutting into a formation 45. The cutter 18 includes a diamond material table 44 affixed to a tungsten carbide substrate 38 that is bonded into the pocket 34 formed in a drill bit blade 14. The drill bit 10 (see FIG. 1) will be rotated for cutting the inside surface of a cylindrical well bore. Generally speaking, the back rake angle “A” is used to describe the working angle of the working surface 20, and it also corresponds generally to the magnitude of the attack angle “B” made between the working surface 20 and an imaginary tangent line at the point of contact with the well bore. It will be understood that the “point” of contact is actually an edge or region of contact that corresponds to critical region 56 (see FIG. 2) of maximum stress on the cutter 18. Typically, the geometry of the cutter 18 relative to the well bore is described in terms of the back rake angle “A.”
In order for the body of a drill bit to be resistant to wear, hard and wear-resistant materials such as tungsten carbide are typically used to form the drill bit body for holding the PDC cutters. Such a drill bit body is very hard and difficult to machine. Therefore, the selected positions at which the PDC cutters 18 are to be affixed to the bit body 12 are typically formed during the bit body molding process to closely approximate the desired final shape. A common practice in molding the drill bit body is to include in the mold, at each of the to-be-formed PDC cutter mounting positions, a shaping element called a “displacement.”
A displacement is generally a small cylinder, made from graphite or other heat resistant materials, which is affixed to the inside of the mold at each of the places where a PDC cutter is to be located on the finished drill bit. The displacement forms the shape of the cutter mounting positions during the bit body molding process. See, for example, U.S. Pat. No. 5,662,183 issued to Fang for a description of the infiltration molding process using displacements.
In addition to bit bodies being formed by infiltrating powered tungsten carbide with, a binder alloy in a suitable mold, a bit body can also be made from steel or other alloys which can be machined or otherwise cut and finished formed using conventional machining and/or grinding equipment. For example, a bit body “blank” may be rough formed, such as by casting or forging, and is finished machined to include at least one blade having mounting pads for cutting elements. The mounting pads may be formed by grinding or machining to include a relief groove.
PDC bits known in the art have been subject to fracture failure of the diamond table, and/or separation of the diamond table from the substrate during drilling operations. One reason for such failures is compressive contact between the exterior of the diamond table and the proximate surface of the bit body under drilling loading conditions. One solution to this problem known in the art is to mount the cutting elements so that substantially all of the thickness of the diamond table is projected outward past the surface of the bit body. While this solution does reduce the incidence of diamond table failure, having the diamond tables extend outwardly past the bit body can cause erratic or turbulent flow of drilling fluid past the cutting elements on the bit. This turbulent flow has been known to cause the cutter mounting to erode, and to cause the bonding between the cutters and the bit body to fail, among other deficiencies in this type of PDC bit configuration.
Other PDC bits known in the art have reduced the turbulent flow caused by the outwardly projected diamond table by including a relief groove formed in the cutter pocket of the bit body. The relief groove reduces the amount of compressive contact between the exterior of the diamond table and the proximate surface of the bit body under drilling loading conditions, thereby reducing the risk of fracture failure of the diamond table, and/or separation of the diamond table from the substrate during drilling operations. Additionally, the PDC cutter may be mounted so that it is substantially flush with the outer surface of the mounting position of the bit body, thereby reducing the amount of turbulent flow created by and outwardly projected diamond table. Thus, relief grooves often reduce diamond table failure, while retaining the benefits of flush mounting of the cutters on the bit body. However, the geometry and dimensions of a cutter pocket with a relief groove are often difficult to control. Additionally, cleaning a pocket with a relief groove requires more work and time.
Displacements are known in the art for forming relief grooves in the cutter pocket of a matrix bit body. U.S. Pat. No. 6,823,952 issued to Mensa-Wilmot, et al. discloses such a conventional displacement configured to form a relief groove in the cutter pocket on the PDC matrix bit body. This patent is incorporated by reference in its entirety. A conventional displacement 102 is shown in FIG. 4. The displacement 102 is a substantially cylindrical body having a selected length indicated by L, a diameter indicated by D and on one end, and a projection 104 having a selected width W. The length L and the diameter D are selected to provide a mounting pad (106 in FIG. 5) on the finished bit body having dimensions suitable to mount a selected cutting element. Typically, the cutting element affixed to the mounting pad (106 in FIG. 5) will be a polycrystalline diamond compact insert. The projection 104 has a substantially cylindrical shape and extends laterally past the exterior surface 102A of the main body of the displacement 102 by about 0.025 inches (0.63 mm). The displacement is affixed to the mold so that the mounting pad is formed to have a recess or relief groove positioned under a diamond table forming part of the cutting element affixed to the mounting pad.
FIG. 5 shows a blade portion of a bit body formed using a displacement, such as shown in FIG. 4. A blade 110 includes thereon a mounting pad 106, having the shape of a displacement. The radius of the mounting pad 106 is determined by the diameter of the displacement. Typically, this radius is selected to match the radius of the cutting element mounted thereon. A relief groove 108 is formed in the mounting pad 106 by having placed the displacement in the mold so that the projection was positioned outward and downward with respect to the blade 110. Shown mounted in the moutning pad 106 is a cutting element 112 consisting of a diamond table 114 affixed to a substrate 116. Typically, the substrate 116 is formed from tungsten carbide or similar hard material. The diamond table 114 can be formed in any manner known in the art for making diamond cutting surfaces for fixed cutter drill bits. The cutting element is typically bonded to the blade 110 by brazing the substrate 116 to the blade 110.
The diamond table 114 extends longitudinally past the surface of the blade 110 by an amount shown at E. The diamond table 114 has a thickness Z which is selected based on the diameter of the cutting element and the expected use of the particular drill bit, among other factors. Diamond table breakage may be reduced efficiently when the depth X of the relief groove 108 is selected so that the relief groove 108 extends back from the surface of the blade 110 at least about 40 percent of that portion (Z-E) of the thickness Z of the diamond table which does not extend past the edge of the blade 110.
While conventional PDC bit bodies have been designed to reduce diamond table failure, the accuracy of designing the cutter pocket has become more difficult, as has cleaning and preparing the pocket.
What is still needed, therefore, is a structure for a PDC bit body which reduces diamond table failure and increases accuracy of designing the cutter pocket.